The present invention is a method for reducing the ionic chloride content of hydroxyl-terminated siloxanes with a molecular sieve effective in removing residual water from the siloxanes. Since ionic chloride strongly partitions into the water phase, ionic chloride is removed along with the water phase. In a preferred embodiment of the present method an aprotic solvent is employed to facilitate water removal from the hydroxyl-terminated siloxanes.
Hydroxyl-terminated siloxanes, for example, hydroxyl-terminated polydimethylsiloxanes and hydroxyl-terminated polymethylphenylsiloxanes are useful as crosslinkers, plasticizers, and intermediates in the silicone industry. These siloxanes are typically prepared by the hydrolysis of dimethyldichlorosilane or methylphenyldichlorosilane respectively. The resultant product is separated from water providing a hydrolyzate mixture comprising cyclic siloxanes. hydroxyl-terminated short-chain siloxanes, and residual water containing ionic chloride. The ionic chloride present in the residual water can facilitate further reaction of the hydroxyl-terminated short-chain siloxanes to higher molecular weight siloxanes, thus reducing the amount of silanol in the hydrolyzate mixture.
From a commercial perspective, it is desirable to provide a hydroxyl-terminated siloxane hydrolyzate with a level of silanol that will remain constant over an extended period of time to provide for shipping and warehousing of the hydrolyzate. Therefore, it is an objective of the present method to reduce the ionic chloride content of a hydroxyl-terminated siloxane hydrolyzate to provide a more stable hydrolyzate. It is a further objective to decrease the ionic chloride content of a hydroxyl-terminated siloxane hydrolyzate without significantly reducing the silanol content or altering the molecular weight distribution of the siloxanes. The inventors have discovered that molecular sieves provide a surprisingly effective means for reducing the ionic chloride of hydroxyl-terminated siloxane hydrolyzates by removing residual water and the ionic chloride entrained therein. The removal of the ionic chloride is effected without altering the silanol content of the siloxanes or the molecular weight distribution.
Molecular sieve zeolites are known to have a high affinity for water and other polar molecules and have been used for drying gases and liquids, Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 15, Third Edition, p. 649, John Wiley and Sons, N.Y.
McEntee, U.S. Pat. No. B1, 127, 598, Certificate Issued Sep. 10, 1985, teaches a process for removing impurities such as biphenyls from impure silanes and siloxanes. The process comprises contacting the impure silane or siloxane with an adsorbent bed selected from the class consisting of a molecular sieve bed and a charcoal bed such that the impurities are adsorbed on the bed.
Huntress et al., U.S. Pat. No. 4,962,221, issued Oct. 9, 1990, describe a process for reducing residual chloride content of polysiloxane fluids. The process comprises contacting the polysiloxane fluid, containing residual chloride, with selected weakly-basic alkaline metal compounds at a temperature less than 100.degree. C. After an appropriate contact time, the polysiloxane fluid is separated from the solid alkaline metal compounds. In a preferred embodiment of the described process, water is added to the process to facilitate removal of residual chloride.
The described art does not recognize that the ionic chloride content of hydroxyl-terminated siloxanes can be effectively reduced by use of molecular sieves to remove residual water contained in the siloxanes. The high affinity of ionic chloride for the water allows the ionic chloride to be removed from the siloxanes along with the residual water.